1. Field of the Invention
The present invention relates to tannins (predominantly natural polyphenolics), especially but not only for use in adhesives for wood and other products, and is particularly concerned with methods of processing tannins, including tannin extraction.
2. Description of the Related Arts
The bark and wood extracts of various commercially important trees contain polyphenolics in the form of tannins which can form condensation products with formaldehyde to thereby act as bases for wood adhesives. Such condensation products have been widely studied particularly with a view to obtaining suitable adhesives for plywood and particle-board. These tannin bark and wood extracts are well known in the art and may be obtained, for example, by extraction from comminuted wood and bark in water in which the tannins tend to dissolve. However, the molecular weights of the polyhydroxy phenols may range from 3.times.10.sup.2 to 3.times.10.sup.8 and difficulties have been encountered primarily due to excessive viscosity of the extracts and the difficulty in obtaining uniformity in product quality.
The lack of uniformity in product quality may arise because sugars and other carbohydrates, as well as other contaminants, may also dissolve in the water while the high molecular weight polyphenolics may not dissolve if the water is cold. The low molecular weight tannin extract may accordingly have a substantial amount of contamination and a low viscosity leading to excessive penetration of the resin into the material being bonded.
There have been various proposals for alleviating these difficulties, the main one of which is to conduct the extraction in hot water, preferably at about 100.degree. C. This increases the solubility in water of the high molecular weight polyphenolics and increases the ratio of polyphenolic material to contaminants such as sugar.
It is important where the tannin is used in adhesives to include the highly viscous extract material in the resin since this contains the higher molecular weight polyphenolics which provide better bonding characteristics of the tannin extract due to their lower contaminant ratio and since excess penetration of the adhesive into the material being bonded may otherwise occur. On the other hand, the tannin extract may become excessively viscous and be difficult to mix and spread.
For example, the yields of tannin from aqueous single stage extraction of one sample of radiata pine (Pinus radiata) bark, carried out at 20.degree., 60.degree. and 100.degree. C., were 2%, 5% and 10% respectively, and viscosities of 40% aqueous solutions of the 20.degree., 60.degree. and 100.degree. C. aqueous extracts were 300 cP, 800 cP and 5000 cP at 25.degree. C. The yields are accordingly low and the known process is not economically favourable for commercialization. Of more importance, the highest yield product is of poor quality because it is very difficult to formulate adhesives from the 100.degree. C. aqueous extracts; the viscosities of the resultant adhesives are too high for either plywood or particleboard bonding processes. For particleboard manufacture a viscosity of approximately 1500 cP is considered to be maximum and viscosities less than 1200 cP are desirable. It is very difficult to produce such adhesives from the 100.degree. C. tannin extract.
One proposal for reducing the viscosity of the bark and wood extracts has been to treat the extracts with a sulphite compound such as sodium sulphite which tends to cleave the polyphenolic chains and also to sulphonate polyphenolics to produce compounds with greater water solubility. However, we have found that the sulphite reaction is non-selective for the higher molecular weight materials so that the low molecular weight polyphenolics are more readily cleaved. For example, when a sample of 100.degree. C. aqueous extract of radiata pine bark is sulphited (25 parts dried extracts, 1.25 parts sodium metabisulphite, 75 parts water, refluxed for 2 hours) the changes in the percentage of the extract falling within various molecular weight ranges are shown in Table 1.
TABLE 1 ______________________________________ Molecular size distribution 100.degree. C. aqueous extracts Fraction by Unsulphited Sulphited molecular weight (%) (%) ______________________________________ More than 10.sup.6 22.4 19.1 10.sup.5 .about. 10.sup.6 5.5 2.4 10.sup.4 .about. 10.sup.5 29.2 26.0 10.sup.3 .about. 10.sup.4 11.5 1.5 Less than 10.sup.3 31.4 51.0 ______________________________________
It is evident that sulphitation results in a substantial increase in the fraction of lowest molecular weight (&lt;10.sup.3) as a result of selective cleavage of the chains in the fractions having molecular weights of up to about 10.sup.6, whereas the percentage of the fraction having the highest molecular weight is little affected. Adhesives made from such sulphited extracts may spread more readily but are frequently of poor quality because their high content of low molecular weight components leads to excessive penetration of the adhesive into the substrate to be bonded. For example, when total aqueous extracts are sulphited, converted to adhesives and used in the production of plywood the quality of the bonds is only B as described in Australian Standard 2754.1-1985 Adhesives for Timber and Timber Products, Part 1 Adhesives for Plywood Manufacture:Standards Association of Australia, North Sydney N.S.W., 1985, which calls up Australian Standard 2098.2-1977 Methods of Test for Veneer Plywood:Bond Quality of Plywood (Chisel Test) Standard Association of Australia, North Sydney, N.S.W., 1977 as the method for assessing quality.
A further proposal for providing relatively low viscosity tannin extracts from bark and wood is made in our Australian patent specification 533791 in which it is suggested that following aqueous extraction of the polyphenolics, any high viscosity material is separated by ultrafiltration and discarded. This process has been found to provide satisfactory extracts for the formulation of adhesives even though the high viscosity extracts have the best bonding characteristics. However, in commerical applications the provision of membranes to achieve the necessary fractionation is costly and the fractionation itself may be difficult to control.